A synthetic rubber-based hot melt adhesive containing a thermoplastic block copolymer as a main component has widely been used as a hot melt adhesive which is used in disposable products such as a diaper and a napkin and is applied to a base material thereof, for example, a nonwoven fabric, a tissue, a polyethylene film and the like.
JP 2004-137297A discloses that a synthetic rubber-based hot melt adhesive containing a styrene-butadiene-styrene block copolymer as a main component can be employed in disposable products. In case of producing disposable products, a film or a nonwoven fabric is coated with a hot melt adhesive and high-speed coating is sometimes used so as to enhance production efficiency of the disposable products. In particular, it is necessary to increase a discharge amount of the hot melt adhesive so as to coat at a high speed in case of performing spiral coating. Therefore, it is necessary to set the pressure of hot air to be blown to a high value. However, in case of the synthetic rubber-based hot melt adhesive of JP 2004-137297A, the hot melt adhesive may be sometimes scattered under the high hot air pressure.
One of means for enhancing production efficiency of the disposable products includes a method in which an olefin-based hot melt adhesive typified by an ethylene-propylene copolymer is applied at a high speed.
WO 2001-096490A discloses that a propylene polymer can be used as a raw material for a hot melt adhesive. An olefin-based hot melt adhesive of WO 2001-096490A develops high adhesive strength in case of bonding a nonwoven fabric and a nonwoven fabric together. However, the adhesive is insufficient for the disposable products because of insufficient adhesion of the adhesive to a polyethylene film.
The disposable products such as a diaper and a sanitary napkin often have a structure in which an absorber constituted by a pulp, an absorbent polymer and the like is wrapped in a tissue and the outside of the tissue is covered with a nonwoven fabric, a polyethylene film and the like. Therefore, it is required for the hot melt adhesive for disposable products to have a strong adhesive strength to the nonwoven fabric and the polyethylene film.
JP 2009-242533A discloses a hot melt adhesive containing a hydrogenated styrene-isoprene block copolymer (SEPS) and polyolefin. The hot melt adhesive of JP 2009-242533A shows satisfactory adhesion to a nonwoven fabric, but shows poor adhesion to a polyethylene film because of rigid design containing no oil component in principle, and thus it is hard to use in disposable products. Furthermore, since the adhesive of JP 2009-242533A contains an amorphous polyolefin including a high-molecular weight component, it cannot obtain a sufficient width in spiral coating and has high viscosity. Therefore, it was not suited for spiral coating to a polyethylene film at low temperature. In order to cope with a complicated coating pattern, it is necessary to decrease the viscosity by increasing a coating temperature. However, when the coating temperature is increased, the film may be melted and the coated hot melt adhesive may shrink due to cooling, resulting in the formation of wrinkles in the film.